There have been great strides made in the study of laser in recent years. In particular, solid lasers and semiconductor lasers that can emit light of wavelengths ranging from the near infrared region to the infrared region are available in the form of high-powered, small-sized laser devices. Such laser devices are remarkably useful as the light sources for light exposure when printing plates are made by direct transfer of digital data from computers or the like to image recording materials.
An image recording material for use in the PS plate of a positive-working mode for lithographic printing used with infrared laser comprises as the essential components a binder resin soluble in alkaline aqueous solutions and an infrared absorption dye, which will be hereinafter referred to as an IR dye, capable of absorbing infrared laser beam to emit heat. At a non-light exposed portion (which will serve as an image portion) in the image recording material, the IR dye serves to inhibit the binder resin from substantially dissolving in an alkaline developing solution as a result of the interaction between the IR dye and the binder resin. At a light exposed portion (which will become a non-image portion), on the other hand, the interaction between the IR dye and the binder resin is diminished by heat generated, which allows the light exposed portion to dissolve in the alkaline developing solution. Thus, a printing plate for lithographic printing can be formed.
However, the above-mentioned PS plate of a positive-working mode for lithographic printing used with infrared laser has the shortcoming that development is so subject to variations of operating conditions that the degree of development can become excessive or insufficient. This problem is caused because the difference between the force that can stop the image recording material from dissolving in the developing solution at the non-light exposed portion (image portion) and the solubility of the image recording material in the developing solution at the light exposed portion (non-image portion) is considered to be still insufficient in light of a variety of operating conditions. Another problem is that the difference between the light exposed portion and the non-exposed portion in the image recording layer of the PS plate becomes attenuated around the area in close proximity of a substrate of the PS plate, which results in poor reproduction of highlight portions. More specifically, the image forming performance of the PS plate for lithographic printing depends upon the heat generated when the surface of the image recording layer of the PS plate is exposed to the infrared laser. The heat conducting through the image recording layer is unfavorably diffused around the area adjacent to the substrate of the PS plate, where the thermal energy used for image formation, that is, used for making the image recording layer soluble in the developing solution is lowered.
No problem has been generated about the latitude for development in the conventional image recording materials for use in the PS plate of a positive-working mode for lithographic printing capable of forming a printing plate through the exposure to UV light. One example of such conventional image recording materials comprises a binder resin that is soluble in alkaline aqueous solutions and an onium salt, quinonediazide compound or the like. In the non-light exposed portion serving as an image portion, the onium salt or quinonediazide compound causes the interaction with the binder resin to inhibit the corresponding portion from dissolving in the developing solution. In the light exposed portion (non-image portion), the onium salt or quinonediazide compound is decomposed by the application of light thereto to generate an acid, which helps to dissolve the image recording material of the light exposed portion in the developing solution. In this case, however, the problem about the reproduction of highlight portions is also generated because of disturbance of light while the UV exposure is imagewise carried out via a lith type film.
In the case of the image recording materials for the PS plate of a positive-working mode for lithographic printing used with infrared laser, an infrared absorption agent or the like does not help to dissolve the alkali-soluble polymer in the developing solution at the light exposed portion, but just works to inhibit the alkali-soluble polymer from dissolving in the developing solution at the non-light exposed portion. Therefore, it is inevitable to use a binder resin that can originally show high solubility with respect to the alkaline developing solution in order to generate a significant difference between the solubility of the non-light exposed portion and that of the light exposed portion in the alkaline developing solution. This will cause the problems that the scratch resistance is lowered and the image recording material for use in the image recording layer becomes unstable before the initiation of development.
To solve the above-mentioned problems, it is proposed to provide a PS plate with a multi-layered image recording layer, which comprises an upper heat-sensitive layer of which the solubility in the developing solution can drastically change at the time of image formation, and a lower layer that is characterized by high solubility in alkaline solutions, as disclosed in JP KOKAI No. Hei 10-250255 (JP KOKAI herein means a publication of Japanese patent application). In addition, as disclosed in JP KOKAI No. 2001-166477, it is proposed to provide an overcoating layer on an image recording layer in the PS plate of a positive-working mode. In this case, the image recording layer comprises a polymer that is soluble in the alkaline developing solution and a near infrared absorption dye, and the overcoating layer formed on the image recording layer is a near infrared-sensitive layer that shows higher alkali resistance than the image recording layer. Further, for example, JP KOKAI No. 2002-182400 has proposed a plate-making process comprising image-wise exposing a positive-working PS plate wherein a lower layer is located on a hydrophilic substrate and the lower layer comprises a water-insoluble and alkali-soluble resin, and an upper heat-sensitive layer is located on the lower layer and the upper layer comprises a water-insoluble and alkali-soluble resin and an infrared absorption dye and exhibits an elevated solubility with respect to alkaline aqueous solutions when heated; and then developing the plate with an alkaline developing solution comprising as main components an organic compound having a buffering action and a base. However, the thermal efficiency at the light exposed portion is still low because of absorption of heat by the substrate such as an aluminum plate, so that the resultant solubility of the light exposed portion in the alkaline developing solution is not satisfactory at the step of development. Then, it is required to ensure the sufficient solubility of the light exposed portion of the image recording material in the developing solution by increasing the alkali content in the developing solution.
However, there occurs the following problem. Even though the image recording layer having a laminated structure as mentioned above is employed for the PS plate, the force to prevent the image recording material at the non-light exposed portion from dissolving in the alkaline developing solution is still insufficient in the case where the alkali content of the developing solution is elevated. If there exists even a slight scratch on the surface of the image recording material at the non-light exposed portion, the image recording material will easily be dissolved in the developing solution, thereby causing a defective image. The above-mentioned problem has not yet been solved.
When consideration is given to the above, there is a limit to the alkali content in the developing solution even though the increase of alkali content in the developing solution is intended to clear the image recording material off the PS plate at the light exposed portion (non-image portion). It has been considered difficult to form sharp images with high contrast without damaging the formed image portion. Therefore, in order to form image portions with higher contrast in the PS plate having such a multi-layered image recording layer as mentioned above and to impart higher scratch resistance to the PS plate, improvements should be proposed from the aspect of the alkaline developing solution that is used to develop the PS plate.
In addition, there is a problem that in a course of development, an infrared absorption dye, a binder polymer and the like are dissolved from the image recording material into a developing solution and insoluble matter originated from these compounds are accumulated and agglutinated to make development sludge, which may be a cause of damaging processing stability. More specifically, such insoluble matter may adhere to a plate during development procedure in making a printing plate, and then image areas of the plate may be impaired. In addition, the insoluble matter is precipitated and deposited in a processing tank and disadvantageously leads to a large labor or cost for maintenance of processing tanks. Accordingly, there is also a need to resolve the problem of the development sludge.